Former Secretary of State Rex Tillerson called the president "undisciplined" and said he "doesn't like to read."

President Donald Trump is a “a man who is pretty undisciplined, doesn’t like to read, doesn’t read briefing reports, doesn’t like to get into the details of a lot of things, but rather just kinda says, ‘Look, this is what I believe,’” former Secretary of State Rex Tillerson told the world on Thursday.

And on Friday, Trump fired back, calling Tillerson “dumb as a rock” and “lazy as hell.”

Tillerson blamed his strained relationship with Trump on the fact they did not share a “common value system” and were “obviously starkly different in our styles.”

“When the president would say, ‘Here's what I want to do, and here's how I want to do it,’ and I'd have to say to him, ‘Well, Mr. President, I understand what you want to do, but you can't do it that way. It violates the law, it violates the treaty, you know. He got really frustrated,” Tillerson said. “I think he grew tired of me being the guy every day who told him he can’t do that and let’s talk about what he can do.”

The president responded in typical — though perhaps slightly more disgraceful — fashion, lashing out at his first secretary of state with a tweet.

Trump fired back on Twitter, saying Tillerson "didn't have the mental capacity needed" as the nation's top diplomat.

"He was dumb as a rock and I couldn’t get rid of him fast enough," Trump said. "He was lazy as hell. Now it is a whole new ballgame, great spirit at State!"